Human vision includes an interaction of the eyes, which receive visual stimuli, and the brain, which creates clear images from the visual stimuli. This interaction is developed during early childhood and is usually complete by 6 to 8 years of age. Amblyopia is the term used to describe the condition when the vision in one eye is reduced because the interaction between the eye and the brain has not developed properly. This condition affects approximately 2-4% of the population. Untreated amblyopia can result in blindness in the affected eye. In fact, amblyopia is the leading reversible cause of blindness in children in the United States. Successful treatment of amblyopia includes early diagnosis.
Several underlying eye defects can lead to amblyopia. Amblyopia can be caused by strabismus, a misalignment in the positioning of the two eyes. Strabismus can cause the eyes to cross in (esotropia) or turn out (exotropia). Amblyopia can be caused by a significant difference between the refractive errors of the two eyes. Refractive errors include nearsightedness (myopia), farsightedness (hyperopia), or astigmatism. Occasionally, amblyopia is caused by other eye conditions such as opacities (cataract).
The American Academy of Pediatrics and other medical professional organizations recommend that children be screened for vision problems at least by age four. In many instances, correction of vision problems is possible through early diagnosis and treatment, but the diagnosis of non-verbal persons, such as infants, pre-school children, stroke victims, or mentally handicapped persons, can be difficult. Non-verbal persons may not realize or be able to communicate a visual problem. For this reason, many children may not have their vision screened before entering school at age five or six.
Several methods and systems exist for examining the vision of non-verbal persons. For example, a common method of screening vision in pre-school children is the use of an eye chart that is positioned at least ten feet from a child to be examined. Alternatively, the chart may be projected onto a surface or screen. An examiner points to optotypes (letters or symbols) that are displayed on the chart while one eye of the child is covered. A vision chart has several disadvantages in screening pre-school children for vision disorders. First, wall charts are not easily portable and require a special room or hallway to use. Secondly, a second examiner is usually required when using a vision chart if the child is too young to read letters or describe the appearance of the optotype symbols, or if the child otherwise cannot verbalize the correct response due to shyness or lack of understanding of the test. As one examiner stands at the chart and points to certain optotypes on the chart, the second examiner holds a second typically smaller chart at a closer proximity to the child. The second chart contains the same optotypes, but in a different arrangement. With one eye covered at a time, the child points to the optotypes on the second chart that correspond with the optotypes that the first examiner points to on the first chart. The second examiner is needed to monitor the child's responses. This method can give an approximate indication of the refractive error in the child's eyes but cannot indicate strabismus or opacities. This method is not effective for very young children.
Alternatively, an autorefractor can be used for detecting refractive errors in non-verbal persons. The autorefractor measures how light is changed by the person's eyes. Autorefractors can be large, costly, fixed units that constrain the person's head in a fixed position or hand-held, less costly, portable units that must be aimed and directed at the person by a trained operator. Fixed autorefractors require a person to assume and hold a fixed position and focus his eyes on the autorefractor. This can be difficult with children and other non-verbal persons. Portable autorefractors require a trained operator who will maintain a precise distance between the autorefractor and the person's eyes. Further, each eye must be tested separately. Autorefractors can be used to perform objective refraction of a child's eyes. The non-verbal person must look at a light emitting from the device, while the operator adjusts the device to focus the lens at the person's eyes, one at a time, thereby determining the prescription of each eye. An autorefractor measures only the refractive error of a person's eyes, and cannot indicate whether the person has a vision disorder such as strabismus or opacities.
Alternatively, a photoscreener can be used for vision screening. A photoscreener can be a fixed or portable device that produces a hard copy photograph or digitized image of a person's eyes as illuminated by a slightly off-axis flash. The photoscreener camera must be directed at a precise distance and direction with respect to the person's eyes and the person must look at the camera. An expert must evaluate the photographs and identify eye defects, such as gross refractive error, strabismus and opacities. Typically, results of a photoscreener exam are delayed until the photograph is developed and evaluated by an expert.
Electronic devices can also be used for vision screening. These typically capture and analyze an image of a single eye. The image-capturing camera must be placed at a roughly known distance from the person. The camera must be centered on a horizontally central landmark of the person, preferably the person's nose. The person is expected to face directly into the camera. Possible presence of opacities or strabismus can be determined by any method known to one skilled in the art. Possible presence of some degree of refractive error is determined by a photorefractive analysis of the models of the eyes found in the image. Inability to find the person's eyes or to analyze an image for any reason requires that the person be reexamined. Confirmation of possible indications of eye disorders also requires that the person be reexamined. False or missed indications of eye disorders are more likely if initial findings are not confirmed with analysis of additional images. Astigmatism cannot be detected from a single image captured in this manner.
Present vision screening technology typically requires the person to hold a fixed position and focus or to verbally interact with the screener. There is a need for a method and article for screening vision defects in non-verbal persons, including vision defects that manifest in early childhood. The method and article should be inexpensive, uncomplicated, and easily performed without special training. The method and article should not require verbal communication with the person, and should not constrain the person. Furthermore, the method and article should be quick, noninvasive, and painless.